Reparative surgery is available for most types of congenital heart defects. Many children who undergo open heart surgery develop postoperative arrhythmias that are sudden, unexpected and life threatening. The incidence of sudden death in patients who have had repairs of tetralogy of Fallot and transposition of the great arteries as well as other complex lesions exceeds 5%. The goals of this project will be to use the electrophysiologic study in these patients 1) to identify the specific types of rhythm disturbances present in patients with various forms of congenital heart disease; 2) to identify which electrophysiologic parameters in any specific group of patients may be used to identify those patients at high risk for developing serious arrhythmias; 3) to investigate and identify electrophysiologic mechanisms, of specific postoperative arrhythmias; 4) to determine effective drug regimens for specific arrhythmias by utilizing the electrophysiologic study; 5) to determine the effects of variations in operative techniques or types of operative repairs of specific congenital heart defects on the development of postoperative electrophysiologic abnormalities and arrhythmias; 6) to determine the effects of variations in timing of surgical repairs of specific congenital heart defects on the subsequent development of postoperative electrophysiologic abnormalities and arrhythmias. Specific areas to be investigated include the switch repair of transposition versus the Senning or Mustard, the early repair of tetralogy of Fallot versus later repair, and the Fontan repair. The current empiric approaches to postoperative arrhythmias have not decreased the incidence of sudden death. A greater understanding of these arrhythmias including a method to identify high risk patients, knowledge of mechanisms, and information regarding the electrophysiologic effects of therapy should provide a more rational and effective approach to the management of arrhythmias in these children. The electrophysiologic methods used in this study will include the use of programmed electrical stimulation to evaluate electrophysiologic function of the sinus and atrioventricular nodes as well as conduction and refractoriness in the atrium, atrioventricular node and ventricle. The vulnerability of the atria and ventricles to develop arrhythmias will be assessed using the stimulation techniques of rapid pacing and premature extrastimuli. Endocardial catheter mapping techniques will be used to assess conduction and activation within the atria and ventricles. Numerous electrophysiologic parameters will be carefully evaluated to determine the most likely mechanism responsible for spontaneous or induced arrhythmias. Electrophysiologic testing in conjunction with drug administration (both oral and intravenous) will be used to evaluate mechanisms as well as to determine effective antiarrhythmic regimens.